1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to laterally diffused metal oxide semiconductor (LDMOS) devices.
2. Related Art
Laterally diffused metal oxide semiconductor (LDMOS) devices are often used as power switches in power management or radio frequency (RF) integrated circuits. In general, LDMOS field effect transistors operate under high voltages (e.g., greater than or equal to 20 Volts), conduct high current of up to tens of Amps while in the on-state, and can be manufacturing using complementary metal oxide semiconductor (CMOS) processes used to manufacture logic devices.
One conventional LDMOS field effect transistor has a lightly doped layer formed near the edge of the gate electrode and coupled to the drain. This lightly doped layer reduces electric field at the edge of the gate, thus creating a high breakdown voltage while in the off-state and decreasing hot carrier injection (HCI) while in the on-state. However, it is undesirable to use conventional LDMOS field effect transistors with an active drift region in power management applications, because of its low breakdown voltage for operation at 20 Volts or greater. While in the off-state, a portion of drain voltage extends under the gate edge and over the thin gate oxide. Usually these thin gate oxides of deep submicron integrated circuits cannot sustain drain voltage of 20V or greater. Hence, the device cannot meet long term reliability requirements when operating at that high voltages. As a result, the device's rated operation voltage becomes significantly lower.
This problem can be mitigated by extending the gate over a field oxide (e.g., a shallow trench isolation), which is thicker than the gate oxide underlying the gate. However, the field oxide is in the middle of the on-state current path and thus, blocks on-state current flow and causes high on-state resistance. In addition, the field oxide can create high leakage due to a mechanical stress near the substrate and field oxide interface.
Hence, a need exists for an LDMOS device that can be used in analog applications and does not have the disadvantages of the prior art.